AXIS™
45XA
Access Floor Terminal Units
Application Data
TABLE OF CONTENTS
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7
Benefits of UFAD System . . . . . . . . . . . . . . . . . . . . . . . . 1
• REDUCED LIFE CYCLE COST
• IMPROVED COMFORT
Basic Concepts of Ventilation . . . . . . . . . . . . . . . . . . . . 3
• TRADITIONAL OVERHEAD AIR DISTRIBUTION
• DISPLACEMENT VENTILATION
• UNDERFLOOR AIR DISTRIBUTION
• TASK AMBIENT CONDITIONING
UFAD System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
• BENEFITS OF A UFAD SYSTEM WITH ZONE
MIXING BOX
• APPLICATION CONSIDERATIONS
• UFAD SYSTEM CHARACTERISTICS
• SUSTAINABILITY (LEED™ POINTS)
PRODUCT OVERVIEW. . . . . . . . . . . . . . . . . . . . . . . . . . 7, 8
Carrier UFAD System with Zone Mixing Box. . . . . . 7
• FLOOR PLENUM
• DIFFUSERS
• UNDERFLOOR SERIES FAN-POWERED MIXING
BOX
• UNDERFLOOR PERIMETER FAN COIL BOX
• FAN-POWERED ZONE-MIXING BOX
• CENTRAL EQUIPMENT
DESIGN PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9-17
Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Zoning Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
General Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Building Load Considerations. . . . . . . . . . . . . . . . . . . 10
• TWO REGIONS
• LOAD ESTIMATING CONCEPTS AND FACTORS
• LOAD ESTIMATING CONCERNS
Central Equipment Selection . . . . . . . . . . . . . . . . . . . . 11
• FAN STATIC REQUIREMENTS
• PRIMARY COOLING AIR QUANTITIES
• COOLING COILS
• ECONOMIZERS
Room Air Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
• DIFFUSER SELECTION AND LAYOUT
• DIFFUSER TYPES AND STLES
Zone Mixing Box Selection . . . . . . . . . . . . . . . . . . . . . . 13
Distribution System Design . . . . . . . . . . . . . . . . . . . . . 14
• PLENUM DESIGN CONSIDERATIONS
• RETURN AIR DESIGN
• DUCTWORK DESIGN
Heating System Design . . . . . . . . . . . . . . . . . . . . . . . . . 15
• CENTRAL SYSTEM VERSUS HYBRID SYSTEM
FOR PERIMETER ZONES
• PERIMETER FAN-COILS WITH LINEAR BAR
GRILLES
• HEATING INTERIORS WITH CENTRAL
EQUIPMENT
Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
• CONTROL DESIGN CONCEPTS FOR UFAD
SYSTEMS
• TYPICAL CONTROL SEQUENCES
Cost Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
• INSTALLATION COSTS
• ENERGY USAGE
• OPERATION AND MAINTENANCE COSTS
SUMMARY OF RECOMMENDATIONS. . . . . . . . . . . . 18
INTRODUCTION
The concept of underfloor air distribution (UFAD) is not
new, however, recent developments in office space usage, sustainable design, and indoor air quality issues have sparked new
interest in this design concept.
Underfloor air conditioning had its start in computer rooms,
at a time when mainframe computers generated considerable
heat and required complex cabling. Access floor systems
allowed plenty of open space to run cabling and provided a
generous pathway to supply large quantities of cooling air
below the intense heat generated by the electronics. The natural
convection currents of warm air rising allowed cool air to enter
low, cool the equipment, and remove the warm air near the
ceiling. While today’s computer rooms no longer require this
type of cooling, the recent interest in access floor systems in
other applications is based on the same needs that made the
UFAD attractive for the old computer rooms: reduced life cycle
cost and improved comfort.
Benefits of UFAD System
REDUCED LIFE CYCLE COST — In the computer rooms
of the past, the large open floor plenum was a convenient space
to run large amounts of cable for power and communications.
Today, the open floor plenum can be used to meet a new need.
The greatest change in office requirements over the past
15 years has been an ever-growing necessity for voice, data and
power connections to every individual workstation. The open
floor plenum makes it easier to provide these connections. A
recent trend in office design makes use of the open space floor
plan to facilitate the relocation of workers into cross-functional
workgroups. The IFMA (International Facility Management
Association) reported in one of its surveys that, on average, an
office worker experiences a move every 51/2 months; this
means that 44% of the workers move within a year. The term
often used for these types of office moves is churn.
The costs associated with these churn moves are some of
the highest costs an owner or tenant may face, largely because
of the need for cabling moves and changes to the HVAC
systems. A UFAD system could greatly reduce the costs
associated with churn moves and help lower total building life
cycle cost.
Manufacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obligations.
New Book 4
PC 201 Catalog No. 04-51450001-01 Printed in U.S.A.
Form 45-1XA Pg 1
605
4-05
Replaces: New
Book 3 3
4
4
Tab 6a 7a
Tab 4AT1 4AT4 4AT5
IMPROVED COMFORT — The UFAD application in computer rooms was based on the principle that warm air rises. In
the computer room, the warm air was generated by hot electrical equipment, but the same principle may be applied to
the warm air surrounding people. The Building Owners and
Managers Association (BOMA) reported in its “What Tenants
Want Survey” that thermal and indoor air quality concerns are
two of the top concerns and least met expectations of tenants.
Traditional overhead air distribution systems are designed to do
a good job of mixing the air in the space and preventing stratification, but they cannot deliver thermal comfort to every
occupant and provide personal ventilation without becoming
cost prohibitive. Overhead systems may require more fan
energy to overcome the static losses that would result from the
mixing airflow patterns required within the space.
Delivery of ventilation air requires sufficient mixing to
assure that the ventilation component is delivered to the occupied breathing region. By applying the principle that warm air
rises, air can be provided below the occupants and discharged
directly into the breathing region at relatively low velocity. As
people warm the air, the air will rise toward the ceiling by
natural convection. People only breathe the air in the region
that extends from the floor to approximately 6 feet above.
Therefore, the space above this region can be treated as a stratified air layer and the load components in this region can be
treated differently. The result is that air provided underfloor can
be supplied at low pressure and the energy used for space conditioning can be reduced. The ventilation air can be provided in
the region where it is needed most, with pollutants moved gently toward the return. Additionally, adjustable diffusers, which
discharge air to small areas in the space, may be provided as
adjustable by the occupant, which would create a perceived
improvement in thermal comfort. The end result is a system
that is more energy efficient, comfortable, and provides better
ventilation and improved indoor air quality.
The application of a UFAD system requires the system
designer to view the project from a different perspective than
other design projects. The UFAD design process requires
special consideration from the schematic stage, through load
estimating, to commissioning. This document is intended to
provide the designer with some basic guidelines. Further
in-depth analysis of all aspects of UFAD design can be found
in the ASHRAE UFAD Design Guide.
A typical UFAD system is shown in Fig. 1.
RETURN AIR AT CEILING
STRUCTURAL SLAB
UNDERFLOOR PLENUM
RAISED FLOOR
SWIRL DIFFUSERS
PERIMETER FAN COIL
PRESSURIZED PLENUM AIR
PERIMETER GRILLES
Fig. 1 — Typical Underfloor Air Distribution System
2
Basic Concepts of Ventilation
TASK AMBIENT CONDITIONING — This system provides airflow from underfloor that is controlled at the workstation. The control is normally accomplished with a small fan
that directs an air jet at the occupant within the workstation.
TRADITIONAL OVERHEAD AIR DISTRIBUTION — A
traditional overhead air distribution system is shown in Fig. 2.
Traditional ducted systems with ceiling level supply have
relatively high discharge velocities to achieve mixing in the
conditioned space. The intention is to fully mix the supply
air with existing air in the space. This results in pollutants
being recirculated throughout the space. Ideally, this mixing
will dilute pollutants in the entire volume of the space with
ventilation air, so that the entire room is a uniform mixture
of old space air and ventilation air, with adequate ventilation
effectiveness. Ventilation effectiveness is the ability of a system to deliver fresh ventilation air into the breathing zone of
the occupants. In order to achieve ventilation effectiveness, a
traditional system often requires that excess ventilation air be
delivered to the space. In addition, heating from overhead and
the location of returns often results in less than the ideal
mixing.
Supply air velocity and ventilation effectiveness are two of
the main design features that distinguish underfloor air distribution from a conventional overhead ducted system, as
described below.
There are three concepts associated with providing floor
level cooling, each with distinctive characteristics.
DISPLACEMENT VENTILATION — A displacement ventilation system discharges air horizontally near the floor at very
low velocities and near laminar flow conditions. The goal is to
use only the buoyancy effects to create air motion within the
space and to maintain the stratification layer above the
controlled region that is not mixed. While this provides excellent ventilation effectiveness in the occupied region, it is not
the optimum solution for thermal comfort. The temperature
variance from head to toe is much greater than the 3 to 5 degree
difference that is accepted as the standard for thermal comfort.
UNDERFLOOR AIR DISTRIBUTION — A traditional UFAD
system uses the floor space between the structural floor and the
access floor as a supply air plenum. A UFAD system uses the
same buoyancy principles as displacement; however, the air is
discharged into the space with adequate velocity and air pattern
required to provide mixing only within the occupied region. In
this system, as well as in the displacement ventilation system, the
goal is to maintain the stagnant region above the breathing
region. The UFAD system provides a more uniform temperature
in the occupied region to stay within the comfort envelope.
UFAD System Overview — The use of the underfloor
plenum in the UFAD system has two major impacts on system
design.
First, since the air is brought in at floor level, a chance exists
for a cold floor or for cold discharge temperatures near the
occupant’s feet. Either condition would be objectionable, so the
air must be provided at a much warmer temperature than
normal air conditioning levels. The accepted level of supply air
in a UFAD system is between 61 F to 65 F. This requirement
affects equipment selection and operation and also impacts air
distribution devices. The second issue is that the large surface
area and mass of the structural slab result in a thermal mass that
shifts load patterns and creates significant amounts of thermal
decay to diffusers located far from the point at which supply air
enters the plenum.
In a UFAD system, the plenum may be configured using
one of two approaches. Under the first method, air is
discharged into the plenum at zero or neutral pressure and fans
near the outlets draw the supply air from the plenum and
discharge it to the space (Fig. 3). Neutral pressure plenums
may have less leakage around floor tiles and plenum penetrations, but fan wiring and energy costs for this system negate
some of the UFAD system’s inherent benefits. Using the
second method, the entire underfloor plenum is pressurized to a
relatively low pressure of approximately 0.10 in. wg.
In the UFAD system with a pressurized plenum, conditioned air is handled using one of two methods. The first
method (Fig. 4) removes the space return air near the ceiling
and the entire volume is returned to the air-handler. This
method requires special treatment by the air-handler to bypass
the required air and condition the required supply amount
without losing control of temperature and particularly humidity
in the space. The Carrier system (Fig. 5) uses a mixing box
located near the space to draw a portion of the return air near
the ceiling and then mix it with the supply air to maintain the
floor plenum pressure and temperature.
Fig. 2 — Overhead Air Distribution System
3
→ Fig. 3 — UFAD System with Zero Pressure Plenum
→ Fig. 4 — UFAD System with Pressurized Plenum and Full Return
→ Fig. 5 — UFAD System with Pressurized Plenum and Zone Mixing Box
605
4
adjustable pedestals, mounted on two-foot centers, that support
the weight of the access floor panels and the loads on the floor
above. The concrete floor panels (two-by-two, approximately
11/8 in. thick) are laid into this grid system. The structural floor
below is normally sealed to provide a moisture and dust barrier.
The primary use of the flooring system is to provide a service
and utility space to run cabling for voice, data and power. As a
result, the floors can be as little as 4 in. over the structural floor
or they may be much higher, depending on the need. When the
floor space is to be used as a supply plenum for air distribution,
floor heights are normally at 12 to 18 inches.
Access floor systems are successfully applied to many types
of buildings in both new construction and renovation. However, when these floor systems are applied to retrofit situations, a
few special considerations are required. Elevator lobbies and
bathrooms that are built on the original structural floor must be
raised to the new floor elevation or ramps must be provided to
move from the structural floor heights to the raised floor
height.
Types of Buildings — Underfloor air systems are not an answer for every building, but may represent significant life cycle
cost savings when applied to the appropriate buildings.
Perimeter zones and spaces such as conference rooms
present some interesting challenges for UFAD systems. These
spaces have loads that vary greatly with time and that may
have air requirements that cannot be handled by 65° F air.
These spot cooling situations can be handled in two ways. A
fan-powered box can be provided under the floor to increase
airflow, or a separate unit can be provided that can use standard
55° F air. A fan coil unit located under the floor can also be
used to meet cooling and heating needs. When 55° F air is
supplied to zone mixing units, the system may also employ
traditional methods used with overhead air distribution, to meet
high load requirements. The Carrier approach uses this hybrid
system (Fig. 6).
Heating along the perimeter also presents challenges for
UFAD systems. Terminal units are frequently added to provide
the necessary heat. One very common method uses a fan coil
unit that contains an electric or hot water heating coil. The fan
coil unit would be placed under the floor near the perimeter and
would be controlled to overcome transmission losses.
BENEFITS OF UFAD SYSTEM WITH ZONE MIXING
BOX — The Carrier approach employs standard air-handling
equipment and uses duct supply temperatures in the normal air
conditioning range, which can best handle the spot cooling
requirements that arise on every project. The air is distributed
in the space through a pressurized plenum with swirl style
floor grilles. The plenum is supplied by a zone parallel fanpowered box with a special DDC (direct digital control) that
modulates temperature and controls plenum pressure. The
primary damper provides the minimum ventilation air to the
zone and maintains plenum pressure. The ECM (electronically
commutated motor) style fan motor in the box modulates to
mix return air and primary air to maintain the required temperature. Heating and cooling requirements for specialized high
load spaces and perimeter zones are handled with conventional
overhead systems, underfloor series fan boxes or fan coil units.
APPLICATION CONSIDERATIONS
Access Flooring Systems — An access floor is a flooring system that is installed above the standard structural floor of the
building. This book describes the Haworth Access Flooring
System (see Fig 7). This flooring system consists of a matrix of
Fig. 7 — Access Flooring System
→ Fig. 6 — Carrier UFAD Design Solution
5
605
New Construction — In new construction, one architectural
benefit to be considered is that ceiling space could be reduced.
Since large supply ducts are not required, the ceiling space can
be reduced or open ceiling space can be used for lighting and
sprinklers, allowing a floor-to-floor height one foot less than
standard. Then for every 12 stories of building, an additional
story could be added with minimal additional cost in the
building shell cost. In buildings of fewer than 12 stories, the
reduction in height would result in a reduction of overall
building shell cost.
In new construction spaces, the best applications are offices
with open plan construction and buildings in which frequent
office changes are anticipated. Owners can benefit from the
reduced costs of moving offices and the required communications and wiring. UFAD systems may also be used successfully
in other types of space with mostly internal loads and open plan
construction, as found in some schools.
In general, small spaces within a building are not well suited
to underfloor air systems. Nor does a UFAD system make good
sense in a building that has a very high ratio of perimeter to
interior space, or buildings with many spaces that have highly
variable loads.
Retrofit Buildings — A UFAD system is a good choice in a
retrofit situation where the building characteristics make
overhead air distribution systems difficult to apply. Recent
trends in downtown revitalization have led to the rehabilitation
of old warehouses and other existing structures into offices.
There may be costs associated with the relocation of elevators
and plumbing connections, however, the savings and ease of
providing the cabling services and air distribution under the
floor may well offset those costs. In addition, the benefits of
improved thermal comfort and indoor air quality may make the
space even more attractive to tenants.
Special Applications — Another good fit for a UFAD system
is in a building with very high ceilings and no place to run
overhead air distribution. Some places of assembly (meeting
rooms, churches, auditoriums) fit into this category. Again,
open plan and internal load concentration are the factors common to successful applications.
UFAD SYSTEM CHARACTERISTICS — A UFAD system
offers several benefits over traditional systems.
Thermal Comfort — A UFAD system provides a higher level
of perceived comfort by allowing the occupant to adjust a
diffuser and achieve a degree of climate control. The system
also offers an increase in actual thermal comfort, since air distribution diffusers provide good head-to-toe temperature variations without disrupting the benefits of a stratification region.
Ventilation Effectiveness — Fresh air supply is introduced to
the space low. As the air warms, the warm air rises and carries
pollutants toward the ceiling (see Fig. 8). Rather than mixing
the air within the entire space, fresh air is provided to the
breathing region. In ASHRAE (American Society of Heating,
Refrigeration, and Air Conditioning Engineers) Standard 62,
Table 6.2, ventilation efficiency of UFAD systems is listed
at 100%. This ventilation method may actually result in
efficiencies greater than 100%, although this has not yet been
documented. In addition, the perceived indoor air quality is
better. Although it is not recommended, total ventilation
airflow potentially could be reduced because of the greater
ventilation effectiveness of the system.
Energy Usage — The lower fan static requirements of UFAD
systems may provide significant energy savings when
compared to overhead air systems. Economizer hours of operation may increase energy savings as well, depending on the
local climate.
Fig. 8 — Thermal Plume Effect
Reduced Life Cycle Costs — Based on the reduction in costs
associated with office churn, UFAD systems may have better
total life cycle costs. In addition, the energy efficient design
helps to reduce the life cycle costs.
Reduced Floor-to-Floor Height — The ability to reduce the
floor-to-floor height by up to one foot per floor can increase
available floor space or reduce shell construction cost. The
reduction in floor-to-floor height is based on reducing or eliminating ceiling plenums. Therefore, the need for ceilings to hide
light fixtures or piping for sprinklers or to help improve room
acoustics must be evaluated.
Improved Productivity and Health — Recent studies have
drawn a positive correlation between productivity and good indoor air quality and comfort.
There are some characteristics of UFAD systems that must
be considered when judging the suitability of the technology
for a particular application.
Cold Floor — One of the most sensitive points of thermal
comfort on the body is the ankle. Control of floor temperature
can be critical in order to avoid complaints resulting from the
radiant effects of a cold floor. When considering floor diffuser
placement, sensitivity to cold drafts must also be addressed.
Code Issues — Although the concept is not new, UFAD technology as it is applied today is relatively new. Building code
officials may not know how to interpret the codes as applied
to the technology and may impose unexpected requirements
for life safety components such as fire resistant materials,
sprinklers, and smoke detectors.
Condensation — The thermal mass effects of the plenum floor
slabs can have a positive effect on sensible load swings, but
care must be exercised when using control schemes that
pre-cool the slab and then allow large quantities of potentially
moist outdoor air to enter the underfloor plenum. Air supplied
to the underfloor plenum must be kept above the dew point
temperature of the floor slab.
Humidity Control — The impact of local climate and part
load control methodologies must be carefully evaluated.
Higher supply-air temperatures have the potential to counteract
the capability of the system to remove latent heat. The design
analysis should evaluate the effects of part load latent control
and control methodology to be sure that room relative humidity
is kept below 60%.
6
requirement is for interior zones to provide individual control
to at least 50% of the occupants. Since UFAD diffusers provide
individual control inherently, the system again is nearly an
automatic qualification for this point. The third set of points
relates to thermal comfort. Since UFAD systems have been
shown to provide a high degree of personal comfort with space
control parameters that fall within the guidelines of the
ASHRAE standard 55 comfort envelope, the system can
qualify for this credit as well.
The UFAD system has proven to be an excellent system to
consider on projects seeking LEED certification. Remember,
however, that certification is based on the total building system
and the points and overall rating for a specific project are dependent on the design of that project.
SUSTAINABILITY (LEED™ POINTS) — There is a nationwide trend toward constructing buildings that are more
environmentally friendly to meet green building or sustainable
design construction standards. The best-known green building
standard is the LEED-NC Rating System® issued by the U.S.
Green Building council (LEED stands for Leadership in Energy and Environmental Design). In addition, two new standards
have been developed that will have an impact on HVAC
design: LEED-CI Rating System® (for corporate interiors) and
LEED-EB Rating System® (for existing buildings). The
LEED (Leadership in Energy and Environmental Design) voluntary rating system provides standards that rate the building
design in 5 categories: Indoor Environmental Quality, Energy
and Atmosphere, Materials and Resources, Sustainable Site,
and Water Efficiency.
This standard sets green building objectives in the 5 categories listed above for which a design can accumulate up to
69 points. The rating system is based on the number of points
awarded and goes from a certified level at 29 to platinum level
at 52. Some prerequisites must be met for certification, but beyond that, the points may come from any of the 5 categories. Of
the total points possible, HVAC related systems account for
40%. The points are based on the entire building as a system,
and it is the entire building that is certified, not a particular product or system. Underfloor air systems have the potential to earn
points in at least seven areas, primarily in the Indoor Environmental Quality and Energy and Atmosphere categories.
The largest points area in the program is for optimizing
energy performance in the category of Energy and
Atmosphere. This credit varies from 1 point for exceeding the
ASHRAE 90.1 energy cost budget (ECB) model by 15% to up
to 10 points for a 60% reduction. In most UFAD projects the
analysis would involve using the ASHRAE energy cost budget
to compare the underfloor system to the energy costs of a VAV
(variable air volume) reheat system.
The UFAD systems may have a distinct advantage in this
area since they incorporate many potential energy savings
features. Lower fan static requirements as the result of low
plenum pressure distribution will reduce required fan energy.
Mechanical cooling requirements and hence equipment size
and part load efficiency may be improved due to the use of
higher discharge temperatures. Economizer usage may be
extended to get more free cooling hours and help reduce part
load energy requirements. In general, UFAD systems have the
potential to show significant energy savings over the required
base system used in the energy budget model.
The UFAD system may also qualify for points under the
Material and Resources category. Within this category 3 points
can be earned for building reuse. If the project involves an existing building, the percentage of building shell reuse would help
obtain these credits. UFAD systems lend themselves very well to
certain retrofit situations where the building is being renovated
to meet the requirements of new technology without destroying
the current building structure. UFAD gives the architect a range
of options to achieve these goals. Also, depending on the location, it is possible that the floor system used would qualify for
the regional credit that requires that the project be within 200 or
500 miles of the equipment manufacturing location.
In the Indoor Environmental Quality category, UFAD
systems have the potential to qualify for points in 3 areas. This
category is the one in which many of the existing certified
projects have relied on points from UFAD systems to achieve
certification. The first set of points associated with this category is in the area of ventilation effectiveness. As noted earlier,
UFAD systems are not rated by ASHRAE as having any
advantage in ventilation effectiveness. However, in the current
LEED documentation, the UFAD system is implied to have
ventilation effectiveness greater than one. As a result, the use
of UFAD systems may be a qualifying point. The second set of
points addresses system controllability, where the LEED
PRODUCT OVERVIEW
Carrier UFAD System with Zone Mixing Box
FLOOR PLENUM — The space between the access floor and
the structural floor is the floor plenum. Using this space as an
air distribution plenum requires some special considerations.
First, the floor panel seams must provide a tight seal to prevent
air leakage from the plenum into the space. In some cases
ductwork is run through the space to special variable load
areas. Also, the space may need partitioning for life safety or
thermal zoning. A typical underfloor plenum is between 12 and
18 inches in height.
DIFFUSERS — Room air distribution is accomplished through
one of two types of floor diffusers. Interior spaces are controlled
through a system of adjustable passive floor mounted diffusers.
The Carrier 35BF-R diffuser consists of an adjustable swirl
plate located in a mounting basket (Fig. 9). The diffuser is
mounted through a hole cut in the concrete floor panel. The
basket catches any dirt or spilled liquids and prevents contamination of the space below the floor. The adjustable swirl plate
allows the occupants to adjust their diffusers to a comfortable
level in their space. The diffusers are designed to achieve
mixing of the approximately 65 F degree supply air within a
very tight radius of the diffuser and at very low velocity. Typical
diffuser airflow is about 60 to 100 cfm per diffuser.
A second type of diffuser is used with underfloor series fanpowered mixing boxes and perimeter fan coil units. The
Carrier 35BF-D rectangular grille is used to provide an air curtain that washes exterior surfaces with warm or cold air
(Fig. 10). These grilles depend on much higher velocities and
can be used in various lengths to match the airflow requirements of the mixing box or fan coil unit. A deflection blade of
either 0 or 15 degrees can be used to provide the required throw
pattern. A version of the rectangular grille can also be installed
in a plenum that has a control damper to provide variable volume control to selected spaces. The rectangular grilles can also
be used to allow return air from above the floor back down to
the underfloor mixing boxes if desired. If used directly in the
pressurized plenum, a standard balancing damper can be added
to adjust airflow.
Fig. 9 — Swirl Plate Style Diffuser — 35BF-R
7
FAN-POWERED ZONE MIXING BOX (45XC) — The 45XC
is a parallel fan powered box with a standard Carrier DDC controller (Fig. 13). One box is used for each zone and is mounted in
a closet or utility space to control the airflow to the underfloor
plenum. The controller is configured for UFAD to control the
temperature and pressure of the zone. The box uses standard
55 F supply air from the air handler and blends the air with return
air from the zone to provide the 63 F discharge air required in the
underfloor plenum. The ECM motor in the box varies fan speed
as required to control plenum temperature by varying the amount
of recirculated air introduced into the plenum, while the primary
air damper simultaneously maintains the plenum pressure by
controlling the amount of primary air introduced into the
plenum. Minimum damper position stops assure that ventilation
air is always provided to the underfloor plenum. Plenum pressure is maintained at approximately 0.1 in. wg.
CENTRAL EQUIPMENT — Since Carrier’s UFAD system
uses a zone mixing concept and air is supplied to the zone mixing box at normal unit design conditions, special air-handling
equipment is not required. Mixing boxes may be used with air
handlers in chilled water or direct expansion type systems or
with many types of packaged VAV equipment as well. Careful
psychrometric analysis should be made of coil entering conditions to be sure the coil will meet the specific leaving air
conditions for entering sensible heat factors that may be higher
than normal. Part load operating conditions in terms of coil
control or staging should be evaluated to be sure room humidity
levels can be maintained below 60% relative humidity. Economizers should be used with all system types where local climate
permits and should use an integrated differential enthalpy type
control. Because of low static pressure requirements, fan sizes
may be lower than conventional overhead systems. Due to
varying air volumes to the zones, the use of variable frequency
drive (VFD) is recommended; the VFD will also improve system efficiency. The use of high-quality filtration is recommended since a prime consideration of the UFAD system is good
indoor air quality.
Fig. 10 — Rectangular Grille
with Plenum — 35BF-D
UNDERFLOOR SERIES FAN-POWERED MIXING BOX
(45UC) — The 45UC unit is designed to fit underfloor in the
space between pedestals and the grids of the access flooring
system (Fig. 11). Sizes range from 280 cfm to 1200 cfm. These
units are used for perimeter spaces, conference rooms and
other spaces with a highly variable load pattern. Typically,
these fan-powered boxes use supply air from the floor plenum
and increase the volume delivered to the room to meet the
variable needs. The same style box can be used when ducted
primary air is required to meet high thermal load requirements.
Heating can be provided with either hot water coils or electric
heaters.
UNDERFLOOR PERIMETER FAN COIL BOX (42KC) —
The 42KC fan coil box can be used for spaces with variable
load patterns and are well suited to addressing heating requirements of perimeter zones (Fig. 12). Available in sizes from 325
to 2800 cfm, these fan coil units also fit underfloor in the space
between pedestals and the grids of the access flooring system.
Heating can be provided with either hot water or electric heat
coils.
Fig. 11 — Underfloor Series Fan-Powered
Mixing Box — 45UC
Fig. 13 — Fan-Powered Zone Mixing Box — 45XC
Fig. 12 — Underfloor Perimeter Fan Coils — 42KC
8
DESIGN PROCESS
• Perimeter zone size may be restricted by ASHRAE 90.1
requirements for exposure zoning and limits of 50 ft of
linear wall per exposure.
• Zone-mixing terminals should be located in vertical
equipment closets. This approach will occupy some
useable floor space, but is a better choice than locating
the units within the plenum. Locating the terminals outside of the plenum makes maintenance easier and minimizes obstructions within the plenum.
• Zones that include conference rooms and rooms with
plants, process or large equipment or solar loads may
have high cooling or humidity loads. The designer
should condition these spaces with traditional VAV terminals using lower temperature primary air (50 to 55 F).
The Carrier hybrid system design will require smaller air
handling equipment and ductwork.
Design Considerations — When designing a UFAD
system, it is important to keep in mind certain differences
between the UFAD system and a traditional overhead mixing
system. Following are key concepts to be considered to avoid
incorporating elements of a traditional system that are not
desirable in a UFAD system.
• Underfloor systems work best when a stratification layer
is established at 6 feet above the access floor. This
creates a partial displacement ventilation effect within
the room and improves ventilation effectiveness, contaminant control, heat capture, and reduces loads within
the space, transferring them directly to the return air.
• Airflows must be closely matched to the loads within the
occupied region so that design requirements do not “over
air” the space, which would disrupt the partial displacement ventilation effect.
• Mixing occurs only within the occupied region and
occurs so efficiently that a 3º to 5º F temperature gradient
is typical, meeting ASHRAE 55 comfort limit.
• Supply airflow should not include excessive safety factors; this will lead to “over airing” the system, resulting
in many occupant discomfort complaints and the loss of
the stratification layer.
General Layout — In terms of general layout, a UFAD
system may be thought of as an inverted conventional overhead system. A UFAD system is similar to a traditional system
in the following ways:
• Central cooling and heating equipment, as well as ductwork mains and branches to zone mixing terminals, can
be nearly identical to traditional overhead systems.
• Zoning occurs in much the same way as with other
systems, but because of the duct mains above and zoned
distribution below, multiple vertical duct drops and
equipment chases must be strategically located around
the floor plate.
The following layout considerations are unique to the UFAD
system design:
• When using zone fan powered mixing boxes above the
access floor, the chases must provide adequate room to
locate the terminal as well as provide required service
clearances (Fig. 14).
• Diffusers are located in the access floor, much closer to
the occupants, so special care must be used in their
layout. Since the UFAD system is a non-ducted system,
relocation of diffusers can be accomplished simply by
swapping floor panels.
Since UFAD systems work best with uniformly loaded thermal areas, high load zones and variable load zones should be
managed with traditional systems, creating an overall hybrid
system that utilizes the best features of both types of systems
(Fig. 15.)
Zoning Decisions — The underfloor air supply plenum
used in a UFAD system requires that the following factors be
considered when making zoning decisions.
• Thermal decay refers to the increase in cooling supply air
temperature resulting from heat transfer to the floor slab
and access flooring. Zone maximum travel distance is
measured from the terminal discharge into the plenum to
the farthest diffuser. Thermal decay will limit the zone
maximum travel distance to 50 to 60 ft. This means that
from a typical core location, the maximum zone size
would be 120 ft by 60 ft, or 7200 sq ft or less.
• Open area zones that have loading differences are hard to
“isolate” from one another without the use of plenum barriers beneath the access floor. However, too many barriers
will limit accessibility and reduce the ease with which
rezoning and utility changes may be made in the future.
• Zones with high heating loads, such as exterior walls
with glazing, may benefit from the use of ducted linear
bar diffusers that deliver the conditioned air to the load
point without significant thermal decay. Ductwork
should be insulated to prevent loss of cooling capacity.
Fig. 14 — UFAD with Zone Mixing Box
9
Fig. 15 — Carrier UFAD Design Solution
Building Load Considerations
TWO REGIONS — The UFAD system depends on establishing two regions: The occupied region and the stratification
region (Fig. 16). Tests have shown that loads that occur in the
upper stratified layer do not affect the occupied region, however, there is no computerized load-estimating program that
can model this condition. The designer must take into account
these two regions when assigning loads. Using too many safety
factors in the occupied region or being overly conservative in
assigning loads may prevent the system from achieving the
benefit of a reduction in airflow requirements for the space.
LOAD ESTIMATING CONCEPTS AND FACTORS — The
following items address those areas that require either manual
output adjustment or computer program input modifications to
allow designers to more closely model UFAD systems and
correctly assign loads and set zone airflows.
• For stratification to benefit cooling loads, floor diffusers
should mix the air within a tight radius and the throw
should not exceed 4 to 5 feet, so that that it will not affect
the height of the stratification layer, which typically
starts at 6 feet.
• Lighting energy is predominantly radiant energy, so only
1/ of total watts should be assigned directly to the upper,
3
unoccupied region. When return air is taken through the
light troffers, a greater portion of the total watts used (up
to 50%) can be assigned to the unoccupied region.
• Exterior wall conductive load is still transferred into the
zone as predominantly radiant (often over 60%), so only
1/ of the unoccupied wall area load should be assigned
3
to the unoccupied region.
NOTE: Lowering the zone ceiling height to the midpoint value
of the unoccupied region would allow most computerized load
programs to automatically assign the loads addressed above to
the return air.
• Transmitted solar loading comes into the space as 100%
radiant, unless inside shading is used. Since manually
adjusted shades cannot be counted on when sizing equipment or setting airflows, do not assign any transmitted
solar load to the unoccupied region.
Fig. 16 — UFAD Design Load Concepts
• Absorbed solar loading becomes over 1/3 convective, so
an appreciable portion of this amount can be manually
assigned to the upper stratified layer, especially if
wall-washing linear bar supply registers and overhead
perimeter return are used in the room air distribution
design.
• People load should always be assigned 100% to the
occupied region.
• Equipment loading should not be assigned to the unoccupied region unless specific return register placement is
used to capture a portion of that load with the return air,
or it is believed that aggressive thermal plumes will
develop and carry a substantial portion of the load
through the stratification layer.
• Infiltration loads always remain within the occupied
region and ventilation loads always remain a part of the
central equipment coil loading.
10
When the loads have been run and certain manual adjustments made, the designer should be able to define two regions:
QOCCUPIED for the lower mixed region, and QUNOCCUPIED for
the region above, which is the stratification layer.
With these loads the designer can then calculate the following required design parameters:
1. Solve for CFM required in the occupied region. (This is
the air supply that must be supplied by the underfloor
supply diffusers and zone mixing box total air flow.)
CFMRoom = QOccupied/(1.1(TRoom –TPlenum–Supply)) Equation 1
TRoom is the thermostat set point for the space
TPlenum-Supply is assumed discharge air temperature in the
plenum
2. Solve for CFM primary air required. (This is the air
supply from the air handler and is used in zone box
selection.)
CFMzone–Box = QOccupied/(1.1(TRoom –TAHU–Supply)) Equation 2
TRoom is the thermostat set point for the space
TAHU-Supply is assumed primary air temperature into the
zone-mixing box
3. Solve for TRETURN in the stratified, or unoccupied region.
(This is the return air temperature to the zone mixing unit
and the AHU before outdoor air mixing occurs.)
TReturn = QUnoccupied/(1.1xCFMRoom) + TRoom Equation 3
TRoom is the thermostat set point for the space
LOAD ESTIMATING CONCERNS
Airflow — Be careful not to “over air” the space. When
working with Equation 1, the space set point temperature,
TRoom, can be increased to be closer to the temperature at the
stratification layer; this will help to more accurately determine
the actual minimum airflow required to meet the occupied
region load. Similarly, in Equation 3, TRoom can be adjusted to
the higher value at the stratification layer, remembering that
with UFAD systems and the partial displacement effect, return
air temperatures are quite a bit higher than the zone set point
temperature.
Floor panel temperature will be midway between room set
point and plenum temperature, creating both a radiant cooling
effect and convective heat transfer into the plenum, lowering
the required room airflow.
Leakage — Leakage is another concern that should be addressed during the design process. Following are leakage
sources that should be considered:
• Supply air leakage out of the pressurized plenum into
unconditioned spaces through construction gaps
• Supply air leakage between the floor tiles into the occupied region
• Radiant and convective cooling by the lower-temperature floor panels
Leakage into the occupied region and radiant and convective cooling from floor panels may result in overcooling the
conditioned space.
Core vs Perimeter — UFAD systems work best with core
areas, open perimeters with limited glass, and similar spaces
that have low load variations. These zones tend to be larger and
constant volume designs will provide the most cost effective
installations with superior IAQ.
When load concentrations increase and/or high load
variability is present, the requirements for airflow delivery to
the space can increase significantly, since UFAD systems are
typicaly designed to provide supply air at a higher than usual
temperature (65 F) in order to provide a more uniform temperature for the occupants. Adding variable volume equipment
would increase system cost. Therefore, perimeter offices with
high glazing areas, conference rooms, and areas with large
equipment loads are examples of zones where use of dedicated
traditional systems make the most sense. The resulting hybrid
design makes the most of both traditional mixing ventilation
systems with lower cooling supply air temperatures and UFAD
benefits of the base design.
Central Equipment Selection — While a UFAD system is thought of primarily as a variation in air distribution
design, there are many particulars that may affect central
equipment selection:
FAN STATIC REQUIREMENTS — Central fan energy should
be less than required in a traditional system because of the low
plenum pressure and reduction in ductwork downstream from
the air terminal. Reduced external static pressure on the central
equipment is an advantage of a UFAD system. Supply ductwork
runs and air pressure drops on the zone terminals leading to
the plenums are little different than conventional systems. The
same is the case with return ducts, but there is always less
than 0.10 in. wg pressure requirement on the plenum and
diffusers downstream from the zone terminals. While this
savings of 0.25 to 0.50 in. wg appears small, when added to the
overall reduction in CFM associated with UFAD’s partial
displacement effect, the result is a significant reduction in fan
motor horsepower.
PRIMARY COOLING AIR QUANTITIES — These quantities can be reduced when potential load credits are realized
from the stratification layer partial displacement ventilation
effect.
COOLING COILS — Temperature and humidity conditions
entering the cooling coil should be checked to be sure that the
cooling coil selected will be able to achieve the required ADP
and bypass factor. In some cases, this may require the use of
larger coils, more coil rows, or lower face velocities.
Part load loss of humidity control can be a significant issue.
A part load analysis should be done to determine if the
coil control methodology being used or the refrigeration
staging will provide sufficient latent heat removal at part load
operation.
ECONOMIZERS — Air or water economizers should be
used whenever possible for the local climate. The use of an
integrated differential enthalpy type is recommend. Use care in
setting up the economizer changeover temperature to be sure
that the 60% maximum space dew point will not be exceeded.
At 75 F room design, this would be an upper limit of 59 F dew
point temperature.
Room Air Distribution
DIFFUSER SELECTION AND LAYOUT — Since the supply air is introduced into the occupied region, it must be
warmer than traditional systems. The temperature of the supply
air should be approximately 61 to 65 F, or only 8º to 13º F
below the usual room cooling set point. In addition, since the
occupant is much closer to the diffuser in a UFAD system, the
diffuser must be a high induction design to quickly even out the
surrounding space temperature and avoid drafts and occupant
complaints. The air must mix quickly into the room and not
project outside the occupied region so that the stratification
layer remains intact. Tests have shown an ideal airflow of
0.6 cfm/sf for good mixing while maintaining the stratification
height at 6 feet and keeping the room temperature gradient
below 5º F (see Fig. 17).
Assuming similar zone loads, the higher supply air temperature of a UFAD system means that larger quantities of air are
needed, resulting in greater numbers of diffusers located in the
access floor. Since the diffusers have low pressure drops and
plenum pressure is very uniform, UFAD designs can be
considered self-balancing at the diffuser.
11
To select a passive diffuser, determine the required room
CFM with Equation 1 above and divide it by the tabulated
airflow for the diffuser at the pressure maintained in the
plenum. This will be the approximate number of diffusers
required. Adjustments may be made to be sure all spaces
receive adequate coverage.
# Diffusers=CFMRoom/(Diffuser_CFMat_Plenum_Pressure) Equation 4
Table 1 — Application of Passive Diffusers
PASSIVE
DIFFUSER
STYLE
Swirl
Floor grilles
UNIFORM
LOADING
Core — open areas & offices
Perimeter — open areas
Large equip. loads, few people
VARIABLE
LOADING
Base loading
Base loading
UFAD Systems will create a high degree of personal
comfort when designed with floor diffusers that provide a
well-mixed region in the lower portion of the room. Warm
return air travels upward through natural convection and is
carried back to the HVAC system. If the diffusers are not
designed properly, the distance the air travels from the diffuser
into the space could be excessive, causing stratified air to circulate back into the occupied space. Improper system design
could result in poor temperature control and the inability to
manage building loads.
The 35BF-R Swirl diffuser throw is a function of both
airflow and ∆T. Quite often, space loads are over estimated.
The difference in estimated and actual space load results from a
combination of factors. The first factor is that the listed amp
draw of equipment located in the space (such as computers and
copy machines) is used to estimate the load. This estimate may
vary from the actual heat generated by this equipment when
used. The second factor is variable occupancy. Early UFAD
systems were designed around 100 cfm/person at a 10° F ∆T
from room to discharge air temperature. System analysis
proved that with many projects, the airflow (cfm) was too
great, which caused over-cooling. This overcooling, combined
with the 10° F ∆T, made it difficult to control humidity. As a result of this analysis, many current project designs have elected
to reduce the supply temperature in the underfloor plenum
from 66° F to 62° F and the supply airflow rate from 1cfm/ft2
to 0.8 cfm/ft2 or less.
The optimal airflow/delivery rate of the 35BF-R is 15° F ∆T
and 80 cfm per diffuser. This tested performance ensures the
ideal throw of 4 to 6 ft within the occupied region. Temperature
and velocity profiles for the 35BF-R are shown in Fig. 18.
Fig. 17 — UFAD Room Temperature Gradient
DIFFUSER TYPES AND STYLES — The Carrier UFAD
system uses passive type diffusers that are designed for use in
pressurized plenum systems. The two styles, described in the
Product Overview section, page 7 are the swirl diffuser
(35BF-R) and the floor grille (35BF-D). Each passive style is
able to move required air quantities at low plenum pressures
(0.05 to 0.10 in. wg) and has very low noise generation values,
usually under NC 20. In fact, a white noise generation system
within the plenum is often recommended for proper acoustic
privacy in open office situations.
Table 1 summarizes the application recommendations for
passive diffusers. Passive type diffusers have an advantage
over active type diffusers in that passive diffusers can be
rearranged easily by just moving floor panels; active types
require moving fan units, power and control wiring and possibly ductwork. The UFAD diffusers should not be used where
liquid spillage potential is high, even though they have a catch
basin for incidental spills, and they should not be placed in
traffic areas where high rolling loads are expected.
12
8’
8’
7’
7’
77.0-78.0
76.0-77.0
6’
6’
75.0-76.0
74.0-75.0
73.0-74.0
5’
5’
Distance
Above
Floor
4’
72.0-73.0
71.0-72.0
70.0-71.0
4’
69.0-70.0
68.0-69.0
3’
3’
2’
2’
1’
Distance From Diffuser Centerline
36" left
18" left
12" left
6" left
Center
6" right
12" right
18" right
36" right
36" left
18" left
12" left
6" left
Center
6" right
12" right
36"right
18" right
1’
Distance From Diffuser Center
Air
Speed,
FPM
8’
8’
175-200
150-175
125-150
7’
7’
100-125
75-100
50-75
6’
6’
25-50
Distance From Diffuser Center
36"left
18" left
12" left
6" left
9" left
1’
3" left
1’
Center
2’
3" right
2’
6" right
3’
9" right
3’
12" right
Distance
Above
4’
Floor
18" right
5’
Distance
Above
4’
Floor
36" right
5’
36"left
18" left
12" left
9" left
6" left
3" left
Center
3" right
6" right
9" right
12" right
18" right
36" right
0-25
Distance From Diffuser Center
Fig. 18 — 35BF-R Diffuser Temperature and Velocity Profiles
Zone Mixing Box Selection — Each zone (thermostat) will require a dedicated air terminal to satisfy that zone
and supply cooling and heating airflow to those areas covered
by the zone. Larger, more uniformly loaded open office interior
zones can be supplied by parallel fan-powered mixing
boxes (PFPMB) all the way up to 3,700 cfm primary airflow
(5,800 cfm mixed airflow). Smaller perimeter zones and
partitioned interior zones can be handled with the same equipment down to 500 cfm primary airflow.
Once cooling loads have been determined, basic terminal
selection can occur, using traditional manual or software methods. The designer should keep in mind the following items
when selecting units for UFAD systems:
• Primary airflow can be supplied by a traditional temperature source (approximately 50 to 55 F) or even from a
low-temperature central system, keeping supply ductwork mains and branches small.
• Primary airflow is the airflow determined with equation
2 (page 11) and is based on the AHU supply-air temperature. Total airflow is the amount calculated by equation 1
(page 11).
• Care must be exercised in the selection of the zone mixing unit to be sure it will provide the total mixed airflow
based on calculated entering conditions to the zone mixing unit. The use of selection software is recommended.
• Although the units are installed closer to the occupied
space, acoustic values should not be a problem if inlet
duct pressure levels are kept low. Acoustic attenuation
options are also available.
• Optional filters on the recirculated air opening are advisable, removing particulate from the return air stream
before it mixes with the primary air.
Control options that meet the sequences of operation will be
required. Control schemes tend to be more complex with
UFAD, making a DDC system a logical choice.
13
Distribution System Design — As stated earlier in
the General layout section, in a UFAD system the ductwork
from the air handler to the zones is essentially the same as any
other all-air system. However, since the air delivery occurs
underfloor, there are routing and design requirements that are
quite different than in a conventional system. One of the major
differences to take into account when designing the air distribution system is that air diffusion occurs within the occupied
region (up to 6 feet above the finished floor).
PLENUM DESIGN CONSIDERATIONS — The following
guidelines can be used in developing a pressurized plenum
design for HVAC use with passive and active diffusers.
• Common plenum depths are 12 to 18 in. from top of slab
to top of floor panel, with most panels being ≤ 2 in. thick.
Practical minimum plenum depth is 8 in. and plenums
deeper than 24 in. offer no HVAC advantages.
• Airflow leakage must be minimized with superior construction sealing of all joints, including overlapping
joints when laying down carpet squares. (Fig. 19.) If
carpet is not used, the floor manufacturer can provide a
sealing strip to use between the floor panels.
• Sealing of concrete floor slabs reduces particulate
generation, although heavier dirt accumulation within
the plenum will not be entrained by the low velocity
airflows.
• Plenum exterior surfaces, both floor and sidewall, should
be insulated to minimize thermal losses, which can affect
the temperature of the supply air delivered to the furthest
diffusers.
• Obstructions within the plenum should be kept to less
than 50% of the plenum depth to avoid affecting airflow.
This is more important with shallow plenums.
• Plenum dividers (available as part of a manufactured
access flooring system) should be provided when zoning
is critical, as part of an architecturally determined smoke
barrier, and to separate the perimeter from the core when
a hybrid solution is used.
• For good acoustic privacy between offices, the floor
panels should weigh at least 10 lb/ft2. Placing a physical
barrier in the plenum along the common wall line
provides additional attenuation.
RETURN AIR DESIGN
Maintaining Displacement Benefits — When designing the
return side of the air distribution system, keep in mind that
there are two key elements of the UFAD system that the return
air design should support. The first is to help create the partial
displacement ventilation effect and the second is to more
directly remove local high cooling loading conditions. Placing
the return registers on the ceiling or high sidewall will combine
with the stratification layer created by the short throw supply
diffusers in the floor to achieve the desired overall upward
vertical flow of the air within the room. When a space has high
cooling loads, such as a copier or a cluster of data terminals,
placing the return directly above the area may capture a large
portion of the heat, depending on the height of the ceiling and
nature of the supply airflow in the immediate area.
Recirculating Return Air — Variations in return airflow occur
when space humidity control is critical. To control the supply
air moisture content during humid summer conditions, cooling
coil leaving air temperatures must be lower than the supply air
temperatures used with UFAD diffusers. To reheat the air
leaving the cooling coil, return air is routed back into the
supply airstream, at the zone air terminal. For the zone terminal
design to work, warm air is usually ducted back from a
ceiling return plenum. Placing ceiling return registers along the
exterior walls brings an added benefit of pulling in any thermal
plumes that develop under high solar loading conditions,
reducing zone cooling load in the summer, and increasing the
first stage heating effectiveness in the winter.
Fig. 19 — Carpet Used to
Prevent Plenum Leakage
DUCTWORK DESIGN — One of the main benefits of a
UFAD system is the elimination of the requirement for
ductwork downstream of the zone terminals. An airflow
plenum underneath a raised access flooring system provides
low-pressure distribution to the room diffusers. All the
ductwork from the air-handling unit to the zone terminals is
laid out and sized using the same methods used on traditional
mixing ventilation systems. This applies to both supply and
return ducts, as well as the outdoor air ductwork.
Vertical Drops — One way in which a UFAD system differs
from a traditional system is in the vertical drop from the ceiling
plenum down into the access floor plenum. Due to the thermal
decay of the supply-air temperature within the plenum, travel
distance must be limited to 50 to 60 ft from the plenum entry to
the furthest diffuser. This will require multiple entry points on
all but the smallest system. If shallower plenums are used and
frequent obstructions are possibilities, more than the usual
number of plenum entry points should be included in the duct
design. Figure 20 shows a floor plate of a typical large office
building with multiple drops supplying each zone mixing box.
Air Highways — One plenum distribution technique used
with UFAD systems is an air highway. Air highways are
installed within the plenum to distribute zone supply air to
multiple points within the plenum. These are typically
provided by the access floor installing contractor and must be
sized in the same way as normal ductwork. Airflow velocity
should be limited to a range of 1,200 to 1,500 fpm in the main
duct and to a range of 800 to 1,000 fpm in dampered branch
outlets. Diffusers should not be installed in the floor panels
above an air highway.
Although air highways may serve a useful purpose, they obstruct the placement of electric, communication and data cables
under the raised access floors, and may also interfere with other
supply air entry points into the plenum. Future rearrangement
of furniture and partitions above the floor may require air highway adjustments under the floor, which negates the efficiency
and savings possible with a raised floor system.
Smoke Partitions and Fire Walls — Whenever plenum areas
become too large, they inevitably cross over partitions or walls
that, in order to meet life safety code requirements, must
extend down through the plenum to the floor slab. The HVAC
zoning design should take into account these lines and avoid
crossing them. If air must be transferred over the partitions, a
hard duct sleeve with the appropriate smoke and/or fire damper
installed per code should be used.
14
Fig. 20 — Floor Plan Chase Locations
Heating System Design
that switches to full heating from the central HVAC equipment
during the peak heating times. When the system switches
from cooling to heating, normal control strategies for the zone
terminals will control mixing of the warmer primary air with
recirculated plenum air and adjust terminal fan speed to control
plenum pressure. Passive diffusers that had been operating
in the cooling mode will operate properly as heating diffusers.
CENTRAL SYSTEM VERSUS HYBRID SYSTEM FOR
PERIMETER ZONES — The heating method chosen to heat
the perimeter zones depends primarily on the magnitude of the
load and whether or not simultaneous heating and cooling is
required. The need for continuous cooling in many interior
zones makes it difficult to switch the central equipment over to
heating. If the wintertime temperatures drop below 40 F and
the building perimeter has more than 25% glass area, increased
supply-air temperatures will be required to counteract the high
heating loads and downdrafts along the outside walls. In either
of these cases, it is recommended that a separate heating
system be designed for the perimeter zones, creating a hybrid
system with UFAD heating being reserved for the interior.
PERIMETER FAN-COILS WITH LINEAR BAR
GRILLES — High heating loads along the outside walls often
require higher supply-air temperatures than interior spaces
need for morning warm-up or offsetting roof loads. The use of
zone fan-coils located in the perimeter plenums is an ideal solution. Using either hot water or electric heating coils allows local boosting of the zone supply air temperatures above those
temperatures obtained by recirculating ceiling plenum air that
is traditionally single-stage heating for light load conditions.
Use of a linear bar grille style diffuser (such as Carrier’s
35BF-CT) is recommended to spread the air out along the
perimeter wall and offset the downdrafts that develop. During
summer cooling this style of grille is also beneficial in treating
the higher cooling loads that are typical along the perimeter of
most buildings.
HEATING INTERIORS WITH CENTRAL EQUIPMENT — Smaller single-story buildings that do not require
simultaneous heating and cooling are often able to use a system
Control Systems
CONTROL DESIGN CONCEPTS FOR UFAD SYSTEMS — UFAD systems rely on proper design and operation
of the controls, as do all systems. Listed below are concepts
that should be incorporated into the design to maintain the
stratification layer and its many benefits:
• Most UFAD systems use pressurized plenums and
passive diffusers, requiring some type of pressurization
control on either the zone mixing box or central
equipment.
• Perimeter zone control with passive diffusers requires
adjustment of the supply air temperature at the air
terminal. This is best accomplished using Perimeter Fan
Powered Mixing Boxes (PFPMB) connected to the zone
ceiling plenum to mix return air with primary air.
• Resetting supply air temperature higher when loads are
light allows additional hours of outdoor air economizer
cooling, especially in drier climates. However, when
economizer cooling is used, special care must be taken
not to exceed the relative humidity level of 60% in the
space. Also keep in mind that economizer cooling may
not meet the spot cooling requirements of high load
spaces.
15
• Nighttime pre-cooling, when performed correctly so that
condensation does not form on the plenum surfaces, can
offset or delay thermal decay of the zone supply air
temperature.
• Thermostat mounting heights are often being set lower
because of ADA (Americans with Disabilities Act),
from 48 to 54 inches above the access floor. The lower
height is actually a benefit for UFAD systems, because
thermostats that are mounted too high will sense
the higher temperatures near the stratification layer,
requiring that the set point be adjusted upwards to
compensate.
• Systems that use Parallel Fan Powered Mixing Boxes to
control the zone primary airflow for ventilation requirements can use Demand Controlled Ventilation (DCV) to
reduce primary airflow in lower occupancy periods,
resulting in additional energy savings.
• Control requirements of ASHRAE Standard 90.1 should
be followed, including setback requirements and optimum start requirements.
TYPICAL CONTROL SEQUENCES — Plenum mixing boxes
with variable speed fans can provide pressure and temperature
control for plenums that serve interior zones and for exterior
zones that use a separate perimeter heating system. Typical operating sequences are shown in Fig. 21 and 22. The sequences
show basic VAV control of the primary air.
The fan provides plenum temperature control by varying
the amount of recirculated air introduced into the plenum,
while the primary air damper simultaneously maintains the
plenum pressure by controlling the amount of primary air
introduced into the plenum. A wall-mounted space temperature
sensor located in the zone will sense load requirements.
As the zone’s cooling load decreases, the fan speed increases,
which increases the amount of recirculated air drawn in from the
ceiling plenum. Simultaneously, the amount of primary air is
reduced to maintain a constant plenum pressure.
The terminal fan will operate whenever the primary air
source is operating.
System Start-up
1. Points 1 to 2 from Fig. 21 indicate that maximum cooling
airflow is established by the user-defined maximum cooling plenum pressure set point until the zone comes under
control at Point 2. The fan operates at minimum speed.
2. From Point 2, the primary airflow is slowed to reduce
plenum pressure until the minimum cooling plenum
pressure set point is reached at Point 3.
Normal Operation (Cooling — Fig.21)
1. Point 3 indicates that the zone temperature is above the
control (heating) set point; the plenum pressure is maintained at the user-defined minimum cooling plenum pressure set point.
2. At Point 4 in Fig. 21, the zone temperature decreases.
Increasing the fan speed to introduce more recirculated
air, while simultaneously reducing the primary airflow,
will increase the plenum temperature. The plenum
pressure remains constant at the user-defined minimum
cooling plenum pressure set point.
3. As indicated by Points 4 to 5, if the zone temperature
continues to fall, the primary air damper will close.
Primary airflow will not fall below the user defined
minimum ventilation set point at 5. (The minimum ventilation set point may be set to zero.) The fan will provide
recirculated air to maintain the plenum pressure set point.
Normal Operation (Heating — Fig. 22)
1. Upon receiving a heating signal from the air handler controller, the heating mode will be in effect automatically.
2. If the zone temperature is above the occupied heating set
point, the primary air damper modulates to maintain the
minimum heating plenum pressure set point at Point 4′.
The fan operates at minimum speed.
3. If the zone temperature falls, the plenum pressure will
increase until it reaches the user-defined maximum heating plenum pressure set point at Point 5′. The control may
be configured to provide constant volume heating for a
constant supply of heated air to the zone.
Cost Considerations
INSTALLATION COSTS — Many owners and developers
have found that raised floor systems with UFAD significantly
reduce costs associated with space reconfiguration. With the
national churn rate now approaching 50%, it is not unexpected
to find that many new office buildings use raised flooring.
Buildings using UFAD systems have certain cost components
that make them more expensive than buildings using traditional systems. There are compensating cost components that
lower the overall price premium that currently exists for this
application. Since recent job costs show that raised floor can
represent a cost increase of from $3 to $5 per square foot, a few
of these other cost components should be examined.
• The raised floor system is the component with the single
largest cost increase. The cost should be justified based
on the benefits to the entire service delivery system
(HVAC, power, voice and data) and should not be evaluated based on the UFAD system alone.
• Local building and fire inspectors may require sprinkler
protection if plenum depth exceeds 18 in. and often
require smoke detectors within the plenum.
• Generally, HVAC costs for central cooling and heating
equipment and ductwork mains are no different for
UFAD systems than for traditional overhead systems.
• Diffuser costs are dependent on the types of diffuser
chosen for the majority of air delivery requirements.
• If a relatively open underfloor plenum can be used,
significant ductwork savings can be realized compared to
the typical overhead air distribution system.
• Controls costs should be similar to traditional systems
with the same zoning requirements.
• Raised floor systems have the potential to reduce slab-toslab heights by as much as 6 to 12 in. per floor. This
reduction is particularly beneficial in mid-rise and highrise markets with zoning code height limitations.
• Testing and balancing savings can be realized from
pressurized plenum designs, which are essentially selfbalancing.
ENERGY USAGE — UFAD systems have the potential to
save energy in comparison to traditional designs. The bulk of
these savings come from reduced fan energy associated with
lower static pressures and overall cfm reductions created by the
stratification layer partial displacement ventilation effect.
Documented energy costs savings remain difficult to predict
because to date there is no energy simulation software that
accurately models UFAD system performance.
OPERATION AND MAINTENANCE COSTS — Office building UFAD systems are quite new, making it difficult to get actual
owning and maintenance cost data to compare against traditional
HVAC systems. Operations and maintenance costs are primarily
replacement costs for equipment and labor expenses associated
with maintaining the HVAC system and responding to occupant
complaints. While many engineers believe that UFAD may
prove to be more costly to service, research suggests that the
frequency of occupant complaints will be reduced when
occupants are given some individual control of their local
environment.
16
Plenum Pressure
(Maximum Cooling)
PLENUM PRESSURE
2
PRIMARY AIRFLOW
5
Maximum Fan Airflow
Warmer
1
4
3
Plenum Pressure
(Minimum Cooling)
PLENUM TEMPERATURE
Cool
Minimum Ventilation
5
FAN
4
Minimum Fan Airflow
OCCUPIED
CONTROL SETPOINT
OCCUPIED
COOLING
SETPOINT
(HEATING &
MORNING WARM-UP)
LEGEND
----------- Air Source Supplying Heated Air
Air Source Supplying Cool Air
Fig. 21 — Carrier UFAD Control Sequence — Equipment Cooling
Plenum Pressure
(Maximum Heating)
5’
Maximum Heating Airflow
Warm
PLENUM TEMPERATURE
PRIMARY AIRFLOW
Minimum Heating Airflow
4’
Plenum Pressure
(Minimum Heating)
PLENUM PRESSURE
FAN
Minimum Fan Airflow
OCCUPIED
CONTROL SETPOINT
OCCUPIED
COOLING
SETPOINT
(HEATING &
MORNING WARM-UP)
LEGEND
----------- Air Source Supplying Heated Air
Air Source Supplying Cool Air
Fig. 22 — Carrier UFAD Control Sequence — Equipment Heating
17
SUMMARY OF RECOMMENDATIONS
1. Do not eliminate the suspended ceiling. Reduce the depth
of the ceiling plenum and use it as a return air plenum,
gaining additional airflow benefits from assigning greater
amounts of cooling load directly to the return air.
2. Keep plenum depths below 18 in. to avoid potential
requirement for sprinklers.
3. Do not use too many zoning barriers in the plenum;
zoning barriers will restrict the ease of future rezoning
and running of service utilities to various points of use.
4. Use multiple vertical drops to avoid the need for air
highways.
5. Limit the underfloor air travel distance from the supply
air discharge to the most remote diffuser to a maximum
of 60 ft.
6. Do not include excess safety factors; airflow rates should
closely match load requirements within the occupied
region to avoid “over airing” the system. The ideal
airflow 0.6 cfm/sq ft. This will provide good mixing in
7.
8.
9.
10.
11.
18
the occupied regions, maintain the stratification height at
6 ft and keep the room temperature gradient below 5° F.
Carefully run loads for both Qoccupied and Qunoccupied, and
determine if load credits can be taken; equipment size
reduction will benefit the job costs.
Select diffusers so that throws are 4 to 5 ft so that the
stratification layer is preserved. Use passive swirls in the
interior and uniformly loaded large perimeter spaces.
Use a hybrid designs in high load/variable load situations.
Combining the UFAD system with a conventional VAV
system will maximize the benefits of the lower supply air
temperatures of the air-handling unit and keep summertime humidity concerns under control.
Place return air inlets over heat producing equipment and/
or at the outside wall when high solar loads exist. This
strategy will maximize benefit of the partial displacement
ventilation effect.
DDC controls should be considered basis of design for
realizing true UFAD benefits.
Copyright 2005 Carrier Corporation
Manufacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obligations.
New Book 4
PC 201
Catalog No. 04-51450001-01 Printed in U.S.A.
Form 45-1XA
Pg 20
605
4-05
Replaces: New
Book 3 3
4
4
Tab 6a 7a
Tab 4AT1 4AT4 4AT5